Recording materials using a combination of an electron donating colorless dye (hereinafter referred to as color former) and an electron accepting compound (hereinafter referred to as color developer) are well known as pressure-sensitive paper, heat-sensitive paper, light- and pressure-sensitive paper, electric heat-sensitive paper, and heat sensitive transfer paper. The details for these recording materials are described, e.g., in British Patent No. 2,140,449, U.S. Pat. Nos. 4,480,052 and 4,436,920, JP-B-60-23992 (the term "JP-B" as used herein means an "examined Japanese patent publication"), and JP-A-57-179836, JP-A-60-123556, and JP-A-60-123557 "JP-A" as used herein means an "unexamined published Japanese patent application").
Further, in order to improve color developability and color sensitivity, various kinds of ether compounds and thioether compounds have been proposed as heat-fusible substances for use in recording materials, as described, e.g., in U.S. Pat. Nos. 4,471,074 and 4,480,052. Any of these known heat-fusible substances, however, have some disadvantages in terms of developed color density, sensitivity, and the like.
Extensive studies on these recording materials have been carried out with the purpose of improving their characteristics such as (1) color density and color forming sensitivity and (2) fastness of a developed color image.
The inventors have investigated numerous color formers, color developers and other components of the recording material paying attention to their solubility in oil or water, partition coefficient, pKa, polarity of substituents, position of substituents, change in crystallizability and solubility when used in combination, and the like.